1. Technical Field
This disclosure relates to a bootstrap circuit and a bootstrap method thereof, and more particularly to the bootstrap circuit and the bootstrap method thereof for charging a bootstrap capacitor quickly.
2. Related Art
Voltage converter application such as drive circuits of a motor, ballast, and a cold cathode fluorescent lamp uses an AC or DC voltage source to generate an AC or DC output with a high-voltage or high-current to drive the load. Mainstream conversion circuits use integrated circuit processed by the HV resistant process to achieve the drive stages of power components, so as to drive externally attached power components or ones on the same chip. In additional to power components in the integrated circuit, other associated control circuit will be integrated as well, so as to reduce the size of the application board and the number of external components and to save the cost. However, in order to correctly drive the power components, usually an external bootstrap capacitor will be attached to the application circuit corresponding to bootstrap circuit, so as to achieve high voltage required by the drive stage of the power component. Operationally, the bootstrap circuit has to charge the boot capacitor in a short time. The design uses an integrated high-voltage transistor or other high-voltage resistant component which can be switched to be on or off by the control terminal, such as a laterally diffused metal oxide semiconductor or a bipolar junction transistor, not limited as described herein, to provide a high-voltage and fast charging path, which at the same time is able to prevent abnormal current intrusion, which can be operationally equivalent to a high-voltage resistant diode.
The above power component can forms a half-bridge output stage circuit, which alternatively outputs high voltage and low voltage, so as to output the high-side driver, which includes the above bootstrap circuit, and to form a low impedance path by the bootstrap circuit combined with the high-voltage transistor to fast charge the bootstrap capacitor when the output stage outputs low voltage. When the output stage outputs high voltage, the high-voltage transistor forms a high impedance, so as to prevent bootstrap circuit from intrusion by electric charges in the bootstrap capacitor. In order to effectively achieve the above functions, the high-voltage transistor generally is an N-channel laterally diffused metal oxide semiconductor or an NPN bipolar junction transistor transverse coupled between the bootstrap capacitor and the charging voltage source, and the control terminal (the gate) is coupled to a circuit with a booster function, so as to effectively turn the channel on.
Refer to U.S. Pat. No. 6,060,948; the drawings of the disclosure disclose a bootstrap capacitor C, a high-voltage transistor LD, a diode D1, and a charging voltage source Vs and so on. Assuming that the forward bias is Vd1 when the diode D1 is turned on, then when the high-voltage transistor LD is turned on, the voltage of the control terminal G is 2*Vs−Vd1, and the voltage of the channel is Vs. Therefore, the voltage difference of the control terminal G and the channel is Vs−Vd1. Assuming the threshold voltage of the high-voltage transistor LD is Vth, then the overdrive voltage is Vs−Vd1−Vth. The design should make the overdrive voltage as large as possible in the voltage allowable range, so as to lower the impedance of the high-voltage transistor LD when the channel is turned on and to fast charge the bootstrap capacitor.
Refer to U.S. Pat. No. 6,075,391, the drawings of the disclosure disclose a bootstrap capacitor C, a high-voltage transistor LD, a diode Z1, a diode Z2, and a charging voltage source Vs and so on. Assuming that the forward biases are respectively Vd1 and Vd2 when the diode Z1 and the diode Z2 is turned on, then when the high-voltage transistor LD is turned on, the voltage of the control terminal G is 2*Vs−Vd1, and the voltage of the channel is Vs−Vd2. Therefore the voltage difference of the control terminal G and the channel is Vs−Vd1+Vd2. Assuming the threshold voltage of the high-voltage transistor LD is Vth, then the overdrive voltage is Vs−Vd1+Vd2−Vth. If Vd1 approximately equals Vd2, the overdrive voltage is Vs−Vth. Compared with U.S. Pat. No. 6,060,948; U.S. Pat. No. 6,075,391 has a larger overdrive voltage, which enhances the charging efficiency of the high-voltage transistor LD. Or in the case of the same efficiency, the design of U.S. Pat. No. 6,075,391 will have a smaller chip area needed by integrated high-voltage transistor LD and thus a lower cost. Therefore, we can know from the comparison of these two patents that the efficiency of the high-voltage transistor could be enhanced if new better designs for further increasing the overdrive voltage of the high-voltage transistor are made.